1. Field of the Invention
The present invention relates to a method for molding plastic compound and an apparatus therefor and more especially it relates to a method for producing plastic compound with excellent dispersion property from a synthetic resin and an additive therefor, and an apparatus for said method.
2. Description of the Prior Art
For example, one existing method for plastic coating comprises a step of coating the surface of a solid substance, for example, metal with liquid resin, and the other existing method comprises a step of coating by heat-fusion bonding powdered resin on the surface. Recently, the latter method has been increasingly used because it affords achievement of uniform coating and application to coating with various kinds of plastic such as high polymer plastic, fusion-resisting plastic and the like which are difficult to be used as coating materials. By the latter method or a powder coating method, a uniform coated film can be obtained on the whole surface, but such a coated film is very thin so that color shading or crazing are apt to occur when pigments or other additives are not uniformly dispersed in a dispersant or a resin. Such dispersion of an additive in a powdered resin is achieved by kneading of the resin and the additive, which kneading comprises repeated application of compressive force and shearing force on material to be treated. However, effective kneading of material cannot be achieved by the prior art by which especially sufficient effective shearing force is not applied to the material.
Powdered coating materials have been manufactured by a conventional single- or multiple-screw extruder with torpedo for extrusion molding or an improvement thereon, but such an apparatus has not been desirably effective. Kneading, which determines dispersion property of the obtained compound, is regarded as an important problem not only in powder coating but in usual molding. Thus, it is not too much to say that kneading will determine the quality of a molded plastic product.
When synthetic resin and additive such as pigment and hardener are preliminarily heated and then kneaded and dispersed at a stroke by means of one or two screws, the kneading and dispersing operations are not effectively carried out so that the synthetic resin material is apt to be solidified around the screw or when the extruded material is powdered in order to carry out painting, air holes of 50 to 60.mu. are apt to remain in the coating. It is quite unclear that the air holes are generated due to either coagulation of the pigment or that of the hardener.
Before the screw extruder is used, the kneading and dispersing operations have been carried out by means of the roll. When the roll is used, the material can be well kneaded and accordingly air holes respectively are made much smaller than those in case of the screw extruder. However, the roll kneading and dispersion apparatus employs a batch system, so that the quality of the products becomes uneven.
Accordingly as required amount of powdered coating material to be made in a roll apparatus increases, the batch system is apparently disadvantageous, thus a continuous screw extruder has been employed instead of the roll apparatus.
The present invention is proposed in order to obtain resin material of plastic resin and additive therefor having good kneading and dispersing property by combining the screw extruder and the roll means.
According to the present invention, the screw extruder for preliminarily and effectively heating, stirring and mixing the plastic resin and the additive therefor is provided. The stirring and mixing operations are carried out preliminarily at a temperature where a solidifying reaction does not occur in order to more effectively perform the kneading and dispersing operations at the next step for the material by high-speed shearing. Therefore, the length of the screw can be made smaller than that of an ordinary screw and the ratio of L/D is less than 16.
Two screw extruder produces a best means to obtain a low and even temperature distribution in the extruder system.
Furthermore, the ordinary screw having the purpose of stirring and mixing is effectively employed. Further, internal pressure applied to the material is low, so that almost of the resin material is prevented from staying in the screw extruder.
According to the present invention, next, semi-molten resin material made by the screw extruder is supplied to a high speed shearing, kneading and dispersing apparatus.
This apparatus of the present mention, called an internal roll system, employ a barrel and a rotor. The rotor rotates inside of the barrel and revolving and shearing force is applied upon the resin material disposed in a narrow clearance between the barrel and the rotor during its rotation and revolution.
The rotor is revolved towards the direction which is opposite to that of its rotation. The position of a minimum clearance is changed or moved around the inner circumference of the barrel.
The resin material is adapted to be equally subjected to shearing force while the resin material progresses from an inlet of the apparatus to an outlet thereof.
According to the tests carried out by the inventor, a minimum clearance between the barrel and the rotor is preferably 0.05 to 0.5 mm. The relative speed on the inner circumference of the barrel between them is desirably 0.5 m/sec to 10 m/sec. The faster the revolving speed of the rotor is and accordingly the faster the rotating speed is so as to maintain said desirable relative speed, the quality of the obtained resin is excellently improved.
Especially, particles of the pigment are smaller than several micron and secondary coagulated in its natural condition. Therefore, it is necessary to disperse the coagulated particles by applying shearing force upon them in the narrow clearance between the barrel and the rotating rotor.
In case that the screw extruder and a high speed shearing, kneading and dispersing apparatus which are respectively independent are combined, the products extruded from the screw extruder and a hardener supplied from another independent apparatus can be supplied together to a high speed shearing grading apparatus.
Thus, the screw extruder can be supplied with the plastic resin material and additive having no hardener. So that the plastic material can be heated, stirred and mixed without solidifying and furthermore the plastic material can be freely extruded because there is not provided a high speed shearing, kneading and dispersing apparatus.
The conventional method for kneading plastic molding compound comprises passing the material through the minimum clearance defined between the outer surface of a screw, a modification thereof or the like disposed in a barrel and the inner surface of the barrel, or defined between a pair of compressing rollers, at said minimum clearance compressive force and shearing force being applied on the material or a resin and an additive. However, said minimum clearance is located in a fixed position with respect to the whole circumference of the inner surface of the barrel or to a predetermined length within the barrel. Therefore, the material passes through said minimum clearance only at low frequency, and at the same time said passing is not smoothly carried out. However, if rotational speed of a rotor is increased in order to promote passing of the material, the temperature of the passing material rises rapidly, thus causing thermal metamorphism of the compound, making shearing ineffective, and resulting, especially in case of thermoplastic compound, in insufficient plastization so that unfused material is mixed into a molded product or the additive is insufficiently dispersed. In addition, if the clearance is made larger, compressive force and shearing force are not sufficiently applied on the material, thus never achieving preferable kneading.
In short, in the conventional method, compressive force and especially shearing force are not effectively applied on the material passing through the minimum clearance portion, so that sufficient kneading cannot be achieved.
From such a viewpoint, development of an apparatus for effectively kneading a synthetic resin and an additive is earnestly desired.